This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The aim of this project is to investigate rhythmic patterns of innate immune system cell activity in zebrafish (Danio rerio). There is increasing evidence that components of the vertebrate immune system are under circadian clock gene control, but the extent to which different white blood cell populations and their activities are influenced is far from clear. Zebrafish are a well-characterized model that is commonly used in the study of molecular mechanism controlling circadian patterns;however, there have been no reports of rhythmic immune activity in this model. Methods used to detect daily patterns of innate immune system activity include respiratory burst, phagocytosis assays, and cell cycle analysis. We have found that phagocytosis and the production of reactive oxygen species by zebrafish leukocytes varied significantly throughout twenty-four hour periods. A distinct peak in cellular ROS levels occurred before dawn, while the kinetics of respiratory burst responses were least rapid at this time of day. Phagocytosis of the gram negative bacteria E. coli peaked late in the day, whereas there was no daily variation in phagocytosis of the gram positive bacteriaS. aureus. As seen in other species, the number of bacteria ingested per cell peaked during the night. These data provide direct evidence of rhythmic immune system activity, and demonstrate that zebrafish can be a valuable model in which to study the relationships between circadian gene expression, systemic pacemakers, and the activity of vertebrate immune system cells.